In conventional mine roof and wall support systems (hereinafter collectively referred to as “mine roof support systems”) a threaded rod-like reinforcing tendon or rebar is imbedded into a bore or drill hole which is drilled into a rock complex. Depending upon the diameter of bar stock used, conventional anchor tendons are generally installed in bore holes which range in diameter from about 25 to 50 mm. Conventional reinforcing tendons are provided with a threaded proximal end which, when the tendon is seated, projects outwardly from the rock face. A nut or other suitable threaded fastener is threaded onto the projecting end of the tendon and tightened against the rock face to compress and consolidate rock forces, and prevent or control ground movement.
Typically, conventional reinforcing tendons consist of a four to eight foot long steel rod which is provided with ribs, or which is otherwise crimped or grooved along its non-threaded length. The ribs or grooves provide a mechanical engagement with anchoring resin or grout used to couple the tendon to the surrounding rock strata in the bore hole.
U.S. Pat. No. 5,873,689 to Mensour et al. which issued 23 Feb. 1999 describes a prior art method and system of securing an anchoring tendon or rebar in a bore hole by the use of one more resin cartridges. Typically, a number of two-part resin cartridges are inserted into the bore hole immediately ahead of the tendon. Following resin cartridge placement, the anchor tendon is slid axially into the bore hole and driven through the cartridges resulting in their rupture and resin mixing. Following mixing, the resin cures and sets. The resulting engagement between the set resin, the sides of the drill hole and the ribbed or crimped portions of the anchoring tendon, fixedly retains the tendon in place. The friction fit created by the encapsulation of the anchor tendon by the resin or grout allows the transfer of load forces which occurs from dilating adjacent rock to the tendon.
In situ testing in mine applications has shown that resin encapsulation along as little as 30 cm of the length of the tendon may be sufficient to achieve breaking strength of a 20 mm diameter rebar under a threshold fracture force. The applicant has appreciated that in fully grouted bolt installations, where the anchor tendon is substantially encapsulated along its entire axial length to a distance of 50 cm or less, and typically within about 25 cm or less from the rock face, the tendon maybe subject to an increased chance of premature failure as a result of differential ground movement. In particular, when ground movement occurs, and dilating rock forces transferred to the tendon exceed the threshold fracture force of the steel, the tendon will tend to break either in the threaded section at the proximalmost end of the bore, where the threading typically produces a reduced diameter portion of the bar, or alternately along mid-portions of the tendon at the dynamic/stable rock interface. Typically, the failure of the tendon results from the overall bar stiffness and the inability of the tendon to plastically deform.
The threaded section of the anchor tendon or rod is generally smaller in diameter than the remaining ribbed section, as a result of the machining process during thread formation. When an anchor bolt fractures or breaks across its threaded end, any connection with mesh screens or other surface retention elements used to capture falling rock and debris is lost. As a result, in conventional anchor tendons, the strength and plasticity of the bar is underutilized, since the steel in the threaded section will begin to deform and yield first.
Furthermore, in underground mine applications, shock forces produced by blasting in initial tunnel formation often produces a fractured or loosened rock zone immediately adjacent to the rock face. The fractured rock zone is typically most pronounced within 1 meter of the rock face, depending upon the particular strata. Frequently, conventional anchors fail across the fractured or loosened rock zone, as a result of differential ground movement between dilating and fixed rock strata. Where an anchor bolt or tendon fractures distally from its threaded end, failure most typically occurs along the portion of the anchor tendon which is laterally adjacent to the fracture rock zone as a result of the bar rigidity, and the inability of the tendon to compensate for dynamic load forces.